Abstract: Of the 1064 patients with essential hypertension,a group of 909 used drinking hard-water from Zaoyuan,Beidaihe,the hardness index of which is 45.21.The composition of Ca^2＋ and Mg^2＋ in the water was 7.70mmol／L and 8.55mmol／L.On the 15th day in the Sanatorium,the average blood pressure of 909 patients decrease 20.25±4.35mmHg SBP)and 14.96±2.92mmHg（DBP）.The remarkable responsive rate was 62.82％,and the total responsive rate 88.34％.The other group of 155 patients used drinking soft-water from both Yang River and Qinglong River.The hardness index of the water was only 6.76 and the composition of Ca^2＋ and Mg²⁺ in the water was 1.51mmol／L and 0.92mmol／L.The average blood pressure decreases in the 155 patients was: SBP 8.35±2.03mmHg and DBP 6.92±1.35mmHg on the 15th day in the sanatorium.The remarkable responsive rate was only 17.42％ and the total responive rate was only 52.90％. The results of the two groups were singnificantly different（P＜0.001）.It is reasonably to assume that the ions of Ca²⁺ and Mg^2＋ in the Zaoyuan drinking water had significant hypotensive effect.